Reactions of olefins with maleic anhydride at elevated temperatures give the corresponding adducts in accordance with the following equation: ##STR1## However, reactions of this type require very long reaction times even at elevated temperatures. In addition, under conventional temperature conditions (200.degree.-300.degree. C.), darkening of the product and formation of black solids are typically observed. These problems are believed to arise from secondary reactions involving maleic anhydride such as, for example, polymerization and decarboxylation. Thus, stabilization of maleic anhydride for these secondary reactions and acceleration of the addition reaction is desirable.
It is known that the reaction of the olefinically unsaturated hydrocarbons with maleic anhydride can be carried out in the presence of a catalytically effective amount of an additive in order to accelerate the reaction. For example, in conventional processes, the reaction times are reduced to a satisfactory level by carrying out the addition reactions in the presence of small amounts, in general from 1 to 5000 ppm by weight, of substances such as furan derivatives (U.S. Pat. No. 4,388,471), iodine (Great Britain Patent Application No. 356,882), bromide (Great Britain Patent Application No. 1,480,453), an .alpha.-bromodialkylketone (U.S. Pat. Nos. 3,953,475 and 3,954,812), hydrogen chloride or calcium bromide (U.S. Pat. No. 3,935,249), a hydantoin derivative (U.S. Pat. No. 3,927,041), p-toluenesulfonic acid (U.S. Pat. No. 3,855,251), a nickel salt (Great Britain Patent Application No. 2,081,274) or a bromophenol (U.S. Pat. No. 4,278,604).
A process for thermally stabilizing acid anhydrides by the addition of an inorganic boron-oxygen compound is taught in U.S. Pat. No. 4,257,958, the teaching of which are herein incorporated by reference.
U.S. Pat. No. 4,599,433, the teachings of which are herein incorporated by reference, is directed to a process for the reaction of an olefinically unsaturated hydrocarbon with maleic anhydride in which an alkoxide of titanium, zirconium, vanadium or aliminum is used as the additive.
In these conventional processes, however, the degree of conversion of the olefin is frequently low. In addition, where halogen compounds are used, there is also considerable danger due to the toxicity of the resulting reaction mixture. Many of these conventional processes also have the disadvantages of product discoloration and formation of solids during the reaction which conaminate the kettle walls or, in more adverse cases, the reaction product. An even more disadvantageous feature is the formation of resin-like residues which render the product useless if it cannot be purified by distillation or filtration.
It has now been found that small amounts of alkoxides of zinc or dialkyl zinc as additives dramatically reduce the formation of black solids and improve product color for the reaction of maleic anhydride and olefinicallyunsaturated hydrocarbons to produce alkenyl succinic anhydrides. The presence of these additives permits the reaction to be conducted at higher temperatures which decreases residence times and allows complete consumption of maleic anhydride to avoid plugging or recycle.